Death of neurons secondary to the misfolding, aggregation and accumulation of specific proteins in the central nervous system (CMS) is a seminal event in the pathogenesis of a number of important neurodegenerative diseases affecting our aging population, including Alzheimer disease, Parkinson disease, and Amyotrophic Lateral Sclerosis (ALS). Some 30,000 Americans currently suffer from ALS-a progressive and universally fatal neurodegenerative disease that leads to loss of both upper and lower motor neurons. While the majority of ALS cases occur sporadically through unknown mechanisms, a smaller percentage are inherited due to dominant mutations in the gene encoding Cu/Zn superoxide dismutase (SOD1). There is overwhelming evidence that mutant (MT) SOD1 leads to motor neuron apoptosis through a gain of one or more toxic properties rather than a loss of enzymatic activity and that a similar buildup of misfolded proteins may underlie sporadic ALS. Many forms of irreversible cell damage trigger the "intrinsic" or mitochondrial cell death pathway, which is tightly regulated at the outer mitochondrial membrane by the pro-apoptotic BCL-2 family proteins BAX and BAK. Mice doubly deficient in Bax and Bak (DKO) die in utero due to insufficient programmed cell death, and cells derived from DKO embryos are strikingly resistant to apoptosis in response to a wide range of intrinsic death stimuli, including protein misfolding crises. Our preliminary data suggest the intrinsic apoptotic pathway is playing a major role in the pathology of familial ALS. Thus, I propose to test the hypothesis that the BAX/BAK apoptotic gateway is the primary conduit through which MT-SOD1 induces neuronal cell death and that blocking this apoptotic pathway will preserve neuronal viability and function despite high levels of this toxic protein. The general strategy is to express MT-SOD1 in motor neurons and transgenic mice genetically deleted for BAX and BAK and test whether this inhibits the ability of this mutant protein to induce neuronal apoptosis and cause ALS. The long-term objectives of this application are to understand how toxic protein species such as MT-SOD1 activate apoptosis in neurons to cause neurodegeneration and to develop an arsenal of small molecules against these apoptotic signals as a therapeutic strategy for preserving neuronal viability in patients with ALS and other neurodegenerative diseases. Two specific aims are outlined: (1) Use an in vitro model to dissect the mechanism of MT-SOD1 induced death in motor neurons, (2) Establish whether deletion of bax and bak preserves neuronal survival and function in an ALS mouse model. The relevance of this application to public health is the identification of a new target (e.g.-intrinsic apoptotic pathway) for the treatment of neurodegenerative diseases.